Dry cleaning of, e.g. non water resistant garments is done with organic solvents.
But the organic solvent alone would not be able to remove water soluble dirt from textiles, unless certain quantities of water and auxiliaries, which form water-solvent-emulsions, are added to it.
The solvent circulates in the machine and is regenerated after each batch cycle. During the regeneration of the solvent, all foreign substances such as the dirt from the cleaned textiles, the auxiliaries and the water must be removed from the solvent as completely as possible. The regenerated solvent is then fed back into the process.
In the past and up until now, low-boiling halogenated hydrocarbons were the most often used solvents. Because of their low boiling points, regeneration by distillation is relatively simple. Most contaminating substances found in the solvent have higher-boiling temperatures than the solvent itself does. Therefore, after vaporization of the solvent in a single-stage batch distillation, the foreign substances can be removed as a residue from the still. This residue is then disposed of. Water, with a boiling temperature that is lower than that of the tetrachlorethene (boiling temperature 121.degree. C.) is vaporized and recondensed together with the solvent. Since it is not miscible with the solvent, it can then be easily separated by deposition.
Because the halogenated hydrocarbons cannot be decomposed by natural biological process, they are as far as possible replaced by paraffin hydrocarbons for reasons of environmental protection. These mediums are decomposed in a few days by natural processes in the soil, as well as in the atmosphere. Since the hydrocarbons are inflammable for safety reasons, a fraction boiling between about 180.degree. C. and about 200.degree. C. is used.
These products have good properties for dry cleaning and can be handled without danger even outside of the machine. Solvent regeneration by vacuum-distillation at an absolute pressure of about 50 mbar to 100 mbar is generally used. Atmospheric distillation would thermally decompose most of the impurities in the contaminated solvent.
The vacuum distillation generally used today is a single-stage batch distillation, principally the same process as is used for distillation of low-boiling halogenated hydrocarbons.
Experience gained in commercial operation shows that as a consequence of the high-boiling temperature of the currently used hydrocarbon solvents, many of the contaminating foreign substances having boiling points between about 130.degree. C. and about 190.degree. C. remain in the distillate, because they vaporize and recondense together with the solvent. The impurities coming from the cleaned goods into the cleaning-solvent contain bad smelling components, such as fatty acids, which accumulate in the solvent from cleaning cycle to cleaning cycle. Since there are also substances present that form an emulsion of solvent and water, water cannot be totally separated from the distilled solvent by sedimentation.
The emulsified water keeps organic compounds dissolved, and in this way additionally bad smelling substances therefore become enriched.
Therefore, it has been found necessary to separate from high-boiling solvents not only those impurities which have a boiling temperature above that of the solvent itself, but also those whose boiling temperature is below that of the solvent itself. Experience shows that in regeneration of high-boiling solvents by single-stage batch distillation, bad smelling substances are enriched and concentrated. Satisfactory removal of these substances is difficult, because even contents of about 0.1% or less in the solvent cause intolerably bad smells. The reciprocal influence of the components on their boiling temperatures (Raoult's Law) increases the difficulty of separation in the stage of vaporization.
The high selectivity of separation which is necessary to remove even very small amounts of impurities from the solvent cannot be obtained by presently existing solvent recovery systems in textile dry cleaning machines.
In the chemical industry, a high separation effect during vaporization is achieved by application of numerous fractionating trays and by a high reflux ratio.
This technology, however, is not suitable for commercial textile dry cleaning, because the equipment would be too large and too expensive.
On the other hand, all the substances of dirt, with higher or with lower-boiling temperatures compared to the solvent itself, even if they are present in a very low percentage, must be removed from the solvent to avoid their enrichment, because a dry cleaning process for human clothes that deposits bad smelling substances on the goods is intolerable.
According to the state of the art, International Patent application PCT/EP93/01361 discloses a fractional vacuum distillation. This process, whose main aim is the recovery of enthalpy of vaporization, is too complicated for commercial use, and the flash evaporator used is not suited to the conditions of textile dry cleaning. The decisive disadvantage, however, is that the separation parameters, such as the condensation temperature of the distillate and the temperature of the vapor state foreign substances escaping the condenser, cannot be adjusted independent of one another. So a sufficient separating-out of small and even varying contents of impurities is impossible. Therefore, the aim of the present invention is a process and device for regenerating a contaminated solvent by removal of the impurities with high selectivity even if the quantity of impurities is very small and variable.